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December 5, 2002

5 Min Read
It’s a first! PET survives e-coat curing

A custom-made PET withstands the deadly heat of a curing oven, racking up another victory for metal replacement.

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In development for only nine months, this grill-opening reinforcement molded from PET (Rynite from DuPont) withstands e-coat oven temperatures without warping, sagging, or distorting.

From the time that Henry Ford instituted the assembly line, automakers have been fine-tuning the carmaking process with an eye toward fewer secondary operations and more streamlined production. Electrocoating, or e-coat, is one such development (see sidebar).

Unfortunately, most plastics can’t make it through the curing oven stage, with its temperatures of 350 to 375F for 30 minutes. This means that plastic parts must be assembled after e-coat, rather than on the assembly line, adding time and cost. For the first time, however, a PET material has taken on e-coat curing and prevailed, thanks to a collaborative effort between Ford, molder Venture Industries (Fraser, MI), and resin supplier DuPont (Troy, MI).

Summer Sweat
Ken Nelson, a senior technical consultant for DuPont, explains that Ford and Venture were looking for a PET material that would withstand e-coat curing temperatures. “Ford had been using sheet molding compound [SMC] regularly to make the grill-opening reinforcement [GOR] for its vehicles. It also used PET for parts installed after the body build and e-coat processes.”

Venture was the molder for these PET parts, and began developing other GORs from another thermoplastic that, ultimately, did not live up to Venture’s expectations.

“We jumped into the project after the second year of the three-year program,” says Nelson, “so the timeline was extremely compressed.” Nine months’ worth of development later, Venture was successfully molding parts.

According to paint, coatings, and chemical supplier PPG Industries (Pittsburgh, PA), electrocoating, or e-coat, is a painting method that uses an electrical current to deposit paint, applying a d-c charge to a metal part immersed in a bath of oppositely charged paint particles. Paint particles are drawn to the metal part and deposited, forming an even, continuous film over the entire surface, until the coating reaches the desired thickness. At that thickness, the film insulates the part, where attraction stops and the process is complete. This highly efficient and automated process allows a shape as large and complex as an auto body to be completely primed in approximately 2 minutes. Hard-to-coat, deeply recessed areas are painted as well, something that is not possible with spray-applied coatings. In addition, the e-coat process delivers as much as 95 percent of every gallon of paint to the part, unlike spray coatings in which large amounts of spray are wasted. The process was developed in 1961.

A majority of the development work was done over a three-month period from June to August, according to Nelson. DuPont created a special grade of its Rynite PET material, and then sent material samples to Ford and Venture in late June. On July 4 (yes, it’s supposed to be a holiday), Nelson and company tested parts in an instrumented e-coat oven he designed.

“We found an area that flexed more than it should, and came up with a rib design that would counteract the tendency. We modified the part without changing the tool and tested it again in the oven. Dial gauge results provided us with a before-and-after comparison that helped convince Ford engineers that the GOR needed the rib addition. In fact, we increased stiffness by 400 percent.”

This latter demo occurred on July 6, and the next day Venture modified the tool to reflect the changes. A moldfilling analysis confirmed that the ribs would also help the part resist shrinkage stresses inherent in all crystalline materials. In addition, the part went from being 10 percent under adequate safety to 70 percent over the safety factor.

Material Makeup
Material capability and part design were the keys to making this application work, Nelson believes. “Material repeatability and properties were crucial, so a good deal of time was spent creating the right polymeric mix.”

PET can vary greatly in crystallinity, and the grade designed for the GOR was aimed specifically at maximum crystallinity and heat resistance. Mineral and glass fillers were added to give the most isotropic properties possible. “We were especially sensitive to making sure the material had enough isotropic behavior to resist warp due to differential shrinkage,” he adds.

While other thermoplastic materials have been e-coated successfully, this industry first has some important differences. “Other parts were not as large and were unrestrained to allow for thermal expansion,” says Nelson. “Here there is almost total restraint of the part, which has to absorb the stresses and return to within specified tolerances.”

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The e-coat-capable GOR appears on 2003 Ford Expeditions and Lincoln Navigators.

Comparisons with other thermoplastic GORs show this PET version to be a breakthrough. A hybrid GOR on the Ford Focus currently in production consists of sheet metal and nylon. However, the all-thermoplastic GOR is the first time PET has been used in such a large part (13 lb, 65 inches long) with sufficiently low expansion. Also, it is a single piece with no metal; designers integrated snapfits and self-locating features. Overall simplicity is also a plus—one material, one mold, no stamping tools, no trimming, no drilling, no milling, and no hole punching as with SMC.

Not only did Ford save weight when compared to the SMC version, but the PET part is also more durable than SMC. It survived a 12° twist test without breakage, and flexed to 24° without breakage as well.

Shortly after Ford received the first parts, it cancelled an SMC backup tool. The GOR was commercialized in March 2002, and appears on the 2003 Lincoln Navigator and Ford Expedition.

Contact information
Venture Industries, Fraser, MI
(586) 294-1500
www.ventureindustries.com

DuPont Automotive, Troy, MI
Ken Nelson; (248) 583-8000
www.dupont.com/automotive

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